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What goes into an LED bulb?

The LED bulb is made using the following parts:

  • Plastic housing with metal cup heat sink inside

  • LEDs

  • LED driver

  • Metal PCB

  • Input and output wires connected to the driver

The LED itself is a semiconductor part which is a Light Emitting Diode. A diode is a semiconductor device which conduct current more easily in one direction than the other. Light emitting diodes emit light when the forward conduction takes place. The colour, or wave length, of the emitted light depends on the band gap of the semiconductor used to make the diode. To make a white LED light, phosphor is used to convert the blue light generated by the LED to white light; blue being the pure wavelength and white being a mixture of so many colours or wavelengths. The LEDs can emit only pure colours. The actual diode chip inside the LED is called a die to which the two terminals are connected , being the anode and cathode. The whole assembly of the Die, leads, phosphor, base are encapsulated in a plastic material which also acts as a covering lens at the top for the desired beam formation. The die itself is cut from a thin wafer of the semiconductor which has thousands of LEDs masked and etched into it. In the case of white LEDs the semiconductor used is InGaN whose band gap yield blue colour light and the forward voltage is around 3 volts. The wafer is grown by an epitaxy process on a substrate of Sapphire or Silicon. This whole LED making process can be divided into two parts, namely the die making and the packaging.

The LED driver is another semiconductor device made from silicon semiconductor material which has hundreds of diodes, transistors and resistors etched inside to make a complete circuit which is usually a non isolated, high power factor, high efficiency, constant current switch mode power supply circuit. This circuit converts the high voltage (220V AC) input, directly, or through a bride rectifier, to a low voltage constant current output for driving the LEDs. If we are using, say 18 pieces of 0.5W LEDs in series in our 9W bulb, the string of LEDs needs 54V DC to be driven correctly.

The LEDs ar soldered onto the metal PCB to make the connections and remove the heat generated by the LEDs out to the atmosphere through the aluminium base of the metal PCB and passing through the heat sink cup and the thin plastic of the housing to the atmosphere. If we did not take this heat out efficiently, the LEDs would get vey hot and soon deteriorate and get dim. The dimness would first arise due to the phosphor deterioration at higher temperature of over 105 degrees C, and finally the semiconductor diode LED itself would get damaged at 175 degrees C.

The wires are used to connect the mains input to the driver and the driver output to the terminals on the metal PCB to power the LEDs.

Arun Bhatia

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